Evaluation Of Fracture Toughness Research Articles

A Study on the Fracture Toughness Evaluation Considering Material Tensile Characteristic for Cryogenic Steel

Abstract ISO 12135, a representative standard for the evaluation of fracture toughness, was revised in 2021. In the previous standard published in 2016, only yield strength was reflected for material property in fracture toughness calculation. In the revised standard, however, yield to tensile strength ratio is additionally reflected in consideration of strain hardening effect of the material. The current fracture toughness calculation formula evaluates fracture toughness reasonably considering the tensile characteristic. However, in some cases of cryogenic steels, fracture toughness is evaluated very conservatively than the previous procedure in 2016. In the case of STS 304L, there is no ductile-brittle transition phenomenon and it has excellent ductility and strength in cryogenic temperature. However, due to the secondary hardening phenomenon, yield to tensile strength ratio is relatively low, and as a result, fracture toughness is evaluated very low. The target of this study is to resolve the undervaluation of fracture toughness of cryogenic steel in the revised ISO 12135. For this purpose, the elastic and the plastic parameters in CTOD equation in ISO 12135 is examined. The crack tip profile is derived by a series of finite element analysis and crack tip opening displacement is calculated for the derivation of new parametric equations for cryogenic materials. The calculated crack tip opening displacement is validated by experimental results.

The importance of fracture toughness evaluation for additively manufactured metals

Yield strength and fracture toughness are key properties of structural materials used by designers for engineering applications, whereas secondary properties like ductility are not generally used in quantitative design calculations. However, since ductility is usually measured along with strength in the same tensile test, it is often used as an indication of the fracture toughness even though these properties are usually not directly proportional. As a result, many researchers initially, and only, screen new materials and processing routes for good combinations of strength and ductility rather than evaluating fracture toughness independently. This can result in poorly informed decisions being made early in the development stages for structural applications where materials with combinations of high strength and good fracture toughness are required. In this article, we evaluate the correlation between fracture toughness and ductility of Al-, Fe-, and Ti-based alloys that were processed using conventional methods as well as by additive manufacturing (AM) routes. We highlight that the correlation between fracture toughness and ductility is very weak, which in the case of AM materials is attributed to their often-metastable microstructures, their layer-by-layer mesostructures, and their anisotropic failure characteristics that lead to pronounced variability in the fracture toughness data for materials with similar ductility. Our findings suggest that an independent assessment of fracture toughness alongside the tensile properties (strength and ductility) is required to correctly optimize the AM processing of structural materials and provide engineers with the properties required for the design of parts and components for structural and safety-critical applications.

Comparative analysis of unloading compliance and normalization methods for fracture toughness assessment in 316L stainless steel at cryogenic temperatures

In the present study, the impact of test temperature, specimen geometry, and evaluation methodologies on the fracture toughness of 316L stainless steel was evaluated. Tensile and fracture mechanics tests were conducted utilizing a cryostat across a range of temperatures from ambient conditions to liquid hydrogen environments. The analysis of crack length prediction and resistance curve was performed using two fracture toughness evaluation methods: the unloading compliance method and the normalization method. The difference in fracture toughness by specimen thickness using the normalization method was within acceptable tolerances for all temperatures. However, fracture toughness evaluation using the unloading compliance method showed apparent negative crack growth at all temperatures, causing differences in J-integrals between specimen thicknesses and discrepancies between physical and estimated crack extensions. The application of the offset technique to the unloading compliance method significantly reduced these differences and made the J-integral of the normalization method and the unloading compliance method agree well. The substantial underestimation in the correction of the rotation angle at ambient temperatures was confirmed through real-time imaging of the crack tip with the unloading compliance method, which contributed to the large deviations between estimated and measured crack extensions.

Enhancement fracture behavior of sustainable cementitious composites using synergy between fly ash (FA) and nanosilica (NS) in the assessment based on digital image processing procedure

The present study has investigated the strength properties and fracture toughness of concrete composites reinforced 5 % nanosilica (NS) by adding different contents of fly ash (FA), i.e.: 0 %, 15 % and 25 %. The evaluated fracture toughness, under three-point flexural loads, were critical stress intensity factor (KIcS) and critical crack tip opening displacement (CTODc). Digital image processing procedure was used in this studies. It was stated that both the 5 % addition of NS and the total share of the nanomodifier with FA grains in the composition of the concrete mix significantly improve the strength properties of concretes, as well as their fracture toughness under first model of cracking in linear and non-linear terms. Moreover, it was observed that in concrete containing only NS the fracture process was clearly delayed and relatively short, whereas crack patterns in this composite were almost perfectly straight with significant crack width. On the other hand, in concretes icorporating NS + FA crack patterns were strongly curved (frequently with branching) with few microcracks in the vicinity of the main cracks. Concretes including FA and NS can be used in the execution of specific types of reinforced concrete structures subjected to fatigue or dynamic loads, e.g. bridge structures or industrial buildings.

Comparative Evaluation of Fracture Toughness and Flexural Strength of Four Different Core Build-up Materials: An In Vitro Study.

To evaluate and compare the fracture toughness and flexural strength of four different core build-up materials. A total of 60 samples were divided into four groups (n = 15) group I: dual cure composite resin reinforced with zirconia particles (Luxacore Z), group II: light cure composite resin (Lumiglass DeepCure), group III: zirconia reinforced glass ionomer cement (GIC) (Zirconomer Improved), and group IV: chemically cure composite resin (Self Comp) respectively. All the core build-up materials were manipulated according to the manufacturer's instructions and poured into the mold. A universal testing machine applied a central load to the specimen in a 3-point bending mode. Fracture of the specimen was identified and the reading was recorded by the universal testing machine. The data were analyzed statistically using one-way analysis of variance (ANOVA) and then compared. Group I showed the highest flexural strength (48.65 MPa) among all the groups while group IV showed the lowest flexural strength (17.90 MPa). Group I showed the highest fracture toughness (99.12 MPa) among all the groups while group IV showed the lowest fracture toughness (36.41 MPa.cm-0.5). When mean flexural strength and fracture toughness values of all four groups were compared by using one-way ANOVA, the compared data was highly significant. Based on the findings of this study, dual cure composite resin was the material of choice in terms of flexural strength and fracture toughness for core build-up material followed by light cure composite resin. The core buildup material serves to strengthen the tooth structure, allowing it to withstand the forces of chewing and preventing the risk of tooth fractures. This material is essential in restoring damaged or decayed teeth, as it provides a stable foundation for further dental work. By reinforcing the tooth structure, the core buildup material ensures that the tooth can function properly and remain healthy for years to come. How to cite this article: Nakade P, Thaore S, Bangar B, et al. Comparative Evaluation of Fracture Toughness and Flexural Strength of Four Different Core Build-up Materials: An In Vitro Study. J Contemp Dent Pract 2024;25(2):191-195.

Modelling of crack propagation in miniaturized and normal SENB specimens based on local failure criterion

The use of miniaturized specimen testing methods is a promising way to solve the problem of limited materials in RPV monitoring programs. The use of miniature specimens allows the evaluation of fracture toughness from other specimen materials used. In particular, the small-size compact tensile test specimen (0.16T CT) is promising for the determination of fracture toughness, as it can be produced from the standard size Charpy specimen that has already been tested. However, if we have only 0.16T CT test, we cannot investigate the dimensional response and also have only one restricted deformation state, which may pose problems in verifying geometry independence and determining local parameters for state-of-the-art analyses. It is therefore recommended to prepare at least two tests with two different restricted deformation specimens. Therefore, the testing of mini single edge notched bending (SENB) is also required and can be worked out from the Charpy specimens. The paper presents the determination of fracture toughness for these miniaturized specimens by modifying the virtual crack closure technique (VCCT) simulation method using GTN parameters instead of energy release as the driving force. This allows the calculation of the J-integral to proceed in parallel with the crack propagation.

Evaluation of fracture toughness according to the pre-strain and temperature effect with H-Section S420 steel for deep sea offshore structures

This study examined the effect of pre-strain on fracture toughness for H-section S420 steel base plates and welds, focusing on the significance of heat-affected zone (HAZ) fracture toughness after pre-strain. Once an offshore structure is installed, it is fixed in the same place for decades, making it essential to secure fracture stability against the damage that might occur owing to sea conditions. In particular, the effect of pre-strain on the fracture toughness of three types of specimens (without pre-strain, 3% pre-strain, and 6% pre-strain) was investigated. Further, an appropriate pre-strain amount in the offshore structure to ensure fracture safety was proposed. The damage suffered by the steel structure decreased for not only the base metal but also the weld joint. The H-section steel satisfied the requirement of critical crack tip opening displacement (CTOD) of up to 6% pre-strain in coarse grain HAZ (CGHAZ). Furthermore, the temperature satisfying the critical CTOD could be predicted using WES 2805.

Comparison and Evaluation of Fracture Toughness of Milled, 3D-Printed, and Conventional Polymethyl Methacrylate: An In Vitro Study

ABSTRACT Introduction: Dentures aim to replicate natural dentition’s esthetics and functions as much as possible. With computer-aided design/computer-aided manufacturing (CAD/CAM) technology, dentistry had a new renaissance with workflow and materials. Aim: The aim is to compare the fracture toughness of the milled, 3D-printed, and conventional polymethyl methacrylate (PMMA) to those processed conventionally. Materials and Methods: 10 CAD MILLED PMMA BLOCKS, 10 3D PRINTED PMMA BLOCKS, and 10 CONVENTIONAL (HEAT CURE) PMMA BLOCKS. Results: A significant difference was seen in the mean flexural module when compared among three study groups as P < 0.05. It was found to be maximum in CAD/CAM PMMA, followed by conventional heat cure and 3D-printed PMMA. Conclusion: Formlabs and Dentca (3D-printed) were significantly weaker in fracture toughness compared to Leucitone 199 (conventional) (P < 0.05). Leucitone 199 (conventional) was significantly weaker in fracture toughness compared to Avadent (CAD CAM) (P < 0.05).

Splitting Mode-I fracture toughness of carbon fibers using nanoindentation

In this study, we conducted a thorough experimental and computational investigation to extend the pillar-splitting technique, developed initially for ceramics, to brittle fibers in reinforced composites. In pillar-splitting methodology, micropillars with circular cross-sections are tested to evaluate the load required to cause instability. The instability load is then correlated with the fracture toughness using the parameter γ which is evaluated using the cohesive zone finite element method (CZ-FEM). In reinforced composites, a unique opportunity arises due to the existence of already present micropillars (reinforcing fibers) for fracture toughness evaluation. In this work, the authors successfully applied the pillar-splitting technique to carbon fibers by investigating the effects of fiber/matrix assembly and the shape of the fiber upon the error in instability load. The minimum height required to avoid matrix effects is found to be equal to the diameter of the fiber reinforcements for a fiber–matrix system with a perfect interface. By contrast, the required protruding height is reduced to zero for a weak fiber–matrix interface. A new parameter, β, is introduced to quantify the deviation of the cross-sectional shape from circularity. An effective γ is then evaluated and used in the pillar-splitting experiments to correlate the instability load to Mode I fracture toughness. Finally, the results are validated experimentally, and the Mode I fracture toughness is evaluated for a range of Toray PAN carbon fibers. Fracture toughness is found to decrease as the modulus of the carbon fibers increases.

Improvement of CMAS resistance of laser glazed and nano-modified YSZ thermal barrier coatings

The present investigation used atmospheric plasma spray (APS) as a method for the production of thermal barrier coatings composed of Yttria-stabilized zirconia (YSZ). A CO2 laser was used to remove surface cracks from the YSZ coatings' surface. Several parameter variations were applied to generate laser-glazed tracks on the coating surface, resulting in an impure surface microstructure with excellent surface properties. Scanning electron microscopy was utilized to analyse the tracks' microstructure. The optimal laser parameters were determined to be 77.7 W laser power, 215 mm laser distance, and, 120 mm s−1 scanning speed based on microstructural characteristics. Once the laser parameters had been optimized, a whole coating surface was laser glazed. In order to ascertain the impact of laser glazing, stability analysis, fracture toughness, and hardness evaluations were conducted on both unglazed and laser-glazed coatings. The growth of a network of cracks was seen on the surface. The surface roughness exhibited a reduction of about tenfold in comparison to the unglazed YSZ. The presence of the monoclinic ZrO2 phase within the coating structure was successfully eradicated. The laser glazing process resulted in enhancements to both the fracture toughness and surface hardness of the coating. Despite the fact that the gaps and imperfections on the coating surfaces were repaired during laser glazing, some cracked structures nonetheless appeared. Nano YSZ was deposited on laser-glazed surfaces by the electrophoretic method in order to eliminate cracks from laser-glazed surfaces. The experimental results obtained from conducting calcium-magnesium-alumina-silicate (CMAS) and hot corrosion tests on laser-glazed coatings indicate that the use of nano YSZ filler leads to a significant reduction in salt penetration during both glassy melt and hot corrosion processes.

Experimental evaluation of fracture toughness of basalt macro fiber reinforced high performance lightweight aggregate concrete

High performance lightweight aggregate concrete has become a popular topic for the development of green concrete due to the incorporation of various industrial wastes to substitute aggregate. However, its brittleness has gradually emerged as an issue with the increasing of compressive strength. To address this concern, the authors in this paper investigated the influence of basalt macro fiber (BMF) on the compressive and splitting tensile strengths of lightweight aggregate concrete at different fiber volume contents and lengths. Moreover, the three-point bending test on notched beams, with the help of acoustic emission and digital image correlation, was also carried out to evaluate the fracture toughness of basalt macro fiber reinforced lightweight aggregate concrete. Finally, the softening constitutive model was determined in the present study. The results showed that the addition of BMF increased the compressive strength of lightweight aggregate concrete and the maximum increase reached 9.03%. As for splitting tensile strength, the maximum increase was 39.88%. In addition, it was observed that the incorporation of basalt macro fiber significantly improved the initial cracking toughness and unstable fracture toughness of plain specimen. The fracture energy of BMF reinforced lightweight aggregate concrete was also increased, and the largest improvement was seen in BMF-0.5–22.5 group, 8.5 times that of plain specimen.

Fracture toughness evaluation of a nuclear graphite with non-linear elastic properties by 3D imaging and inverse finite element analysis

Effective small specimen tests are needed to obtain fracture toughness and elastic properties as the limited availability of irradiated graphite restricts the quantity and dimensions of test specimens. Both properties have evaluated simultaneously in a crack propagation test with the double cleavage drilled compression (DCDC) specimen geometry of a fine-grained graphite (SNG742) that has non-linear elastic properties in the unirradiated condition. Three-dimensional displacement fields were obtained by digital volume correlation of in situ laboratory X-ray computed tomographs, and the 3D crack geometry (crack tip position, crack opening displacements and angle) was determined objectively by a wavelet variance method. The tensile softening of the Young modulus was determined by inverse analysis of the strain field using the finite element model updating (FEMU) method. The strain energy release rate of the quasi-static propagating crack was calculated using the contour integral method in a finite element model with the derived non-linear elastic properties and the measured displacements as boundary conditions. The critical strain energy release rate was constant with crack length (118 ± 14 J m−2) and equivalent to a fracture toughness of 1.13 ± 0.07 MPa m1/2.

Edge notched disc test for evaluation of mode-I fracture toughness of brittle material

In the current investigation, the fracture toughness of plexiglass specimens was accurately measured employing Edge Notched Disc (END) type specimens. Besides, the Notched Brazilian Discs (NBD) were used in order to validate the results of the conducted END experiments. Moreover, a numerical analysis was carried out on the END tests to verify the accuracy of the measured fracture toughness values. In the first stage, the splitting tensile test was used to accurately calibrate the micro-scale properties of the Flat Joint (FJ) model. In the second phase, the numerical simulation of END and NBD models was created, in which the notch lengths in END and NBD were set to 5, 15, 25, 40 and 10, 20, 30, 40, 50, 60 in millimeters, respectively. According to the obtained results, the considered FJ model is capable of making an accurate determination of the potential crack growth path and crack initiation stress like the experimentally obtained results for plexiglass specimens. It was also derived that lengthening the notch could not have a considerable effect on the fracture toughness of the models. The Tensile stress distribution on the failure surface of END test could be the main cause of the lower fracture toughness levels of END test. Besides, the same fracture extension patterns were seen from the experimental investigations and numerical simulations.

Experimental and Numerical Fracture Toughness Evaluation of Spiral Seam Weld of API X65 Steel

In this research fracture toughness of spiral seam weld of API X65 is measured through the ASTM E1820 standard recommendations. According to thin-wall pipe of API X65 in this research, specimen sizes could not satisfy the plain strain condition of ASTM E399 therefore, indirect method is accomplished by evaluating the crack tip opening displacement. According to the nature of welding procedure and inhomogeneous probable defects in the different regions of seam weld, δ-R curve with multi-specimen technique of single-edge notched bend (SENB) specimens is applied for test procedure in this research. The measured CTOD and K1C are 0.23 mm and 265 MPam3/2 respectively. Furthermore, Three-dimensional finite element method numerical evaluation of crack propagation with GTN model has been performed. Good agreement with the other researches was observed for CTOD and fracture toughness of tested material. The difference was about 10 percent.

Experimental evaluation of fracture toughness of bamboo fiber reinforced high performance lightweight aggregate concrete

High performance lightweight aggregate concrete has become a popular topic for the development of green concrete due to the incorporation of industrial waste, e.g., fly ash and boiler slag for aggregate production. However, its brittleness has gradually emerged as an issue with the increasing of strength. To address this concern, the authors in this paper investigated the influence of bamboo fiber on the compressive and splitting tensile strengths of high performance lightweight aggregate concrete at different fiber volume contents and lengths. Moreover, three-point bending test equipped with acoustic emission and digital image correlation on notched beams was also carried out, to evaluate the fracture toughness of bamboo fiber reinforced high performance lightweight aggregate concrete. Finally, the softening constitutive models at different fiber volume contents and lengths were obtained in the present study.

Performance Test of Weld Zone of Rotary Friction Welding of Titanium Alloy Drill Pipe

With the development of complex oil and gas reservoirs, it is difficult for traditional carbon steel to meet the requirements of drill pipe materials in drilling process. Titanium alloy has gradually become a new choice of drill pipe material because of its low density and good corrosion resistance. Titanium alloy has poor weldability and weld zone of titanium alloy drill string is easy to become initial defect and lead to drill pipe failure. Therefore, it is essential to evaluate the performance of titanium alloy drill pipe weld zone. In this paper, the weld zone of titanium alloy rotary friction welding is tested. Metallographic structure, hardness, tension and impact mechanical tests, fracture toughness evaluation and bending fatigue test are carried out. The test results show that the metallographic structure of weld zone changes under the influence of temperature, resulting in different mechanical properties of weld zone from that of pipe body zone. The yield strength and tensile strength in the weld zone are higher than those in the pipe zone, but the fracture toughness is lower than that in the pipe zone. The fatigue life of specimens in weld zone is more than 107 in drilling mud environment. In general, the mechanical properties of weld zone meet drilling requirements.

Fracture toughness evaluation of ground granulated blast furnace slag concrete using experimental study and machine learning techniques

The relationship between mix design and fracture properties of ground granulated blast furnace slag (GGBFS) concrete is complex and influenced by many factors, such as mixture composition. This study evaluates the fracture toughness under mode-I, mode-II, and mixed-mode, performed on centrally straight-notched Brazilian discs. An experimental program generated an extensive database of the fracture toughness of GGBFS mortar, consisting of 143 specimens with different mix designs under different loading angle conditions. Then, a combination of support vector regression (SVR) algorithm and genetic algorithm (GA) as hyperparameter tuner was conducted to develop models for fracture toughness. The SVR algorithm was trained with Radial Basis Function (RBF), polynomial, sigmoid, and linear kernel functions. Based on the Taylor diagram, the models demonstrated robust performance in predicting fracture toughness. The performance metrics indicate that the RBF function is the most reliable model, with R2-score 0.95 and 0.94 for KIC and KIIC, respectively. The importance score of input features was determined with the permutation importance function. The angle between the crack and loading direction is the most influential parameter in the models.

Investigation of microstructural failure in the human cornea through fracture tests

Fracture toughness of the human cornea is one of the critical parameters in suture-involved corneal surgeries and the development of bioengineered mimetics of the human cornea. The present article systematically studied the fracture characteristics of the human cornea to evaluate its resistance to tear in the opening (Mode-I) and trouser tear mode (Mode-III). Tear experiments reveal the dependency of the fracture behavior on the notch size and its location created in the corneal specimens. The findings indicate lamellar tear and collagen fiber pull-out as a failure mechanism in trouser tear and opening mode tests, respectively. Experimental results have shown a localized variation of tear behavior in trouser tear mode and indicated an increasing resistance to tear from the corneal center to the periphery. This article demonstrated the complications of evaluating fracture toughness in opening mode and showed that the limbus was weaker than the cornea and sclera against tearing. The overall outcomes of the present study help in designing experiments to understand the toughness of the diseased tissues, understanding the effect of the suturing location and donor placement, and creating numerical models to study parameters affecting corneal replacement surgery.

Experimental Evaluation of KIEAC of a Carbon Steel Using the Pin-Loaded SENT Geometry

Abstract The experimental determination of the threshold stress intensity factor for environment-assisted cracking (KIEAC) is described by several standards, which allow the use of different specimen geometries and methodologies for crack length estimation. In some cases, the combination of structure, specimen size, or both, main loading direction, and crack orientations of the component that need to be characterized limit the use of standardized geometries. Consequently, alternative geometries must be used. In this study, fatigue pre-cracked pin-loaded single edge notched tension specimens as defined by BS 8571:2018, Method of Test for Determination of Fracture Toughness in Metallic Materials Using Single Edge Notched Tension (SENT) Specimens, were applied for the experimental evaluation of the KIEAC of a carbon steel. The specimens were tested in deaerated substitute ocean water solution saturated with carbon dioxide at 40°C and at 1 and 10 bar (100 and 1,000 kPa) under constant load conditions with incremental (step) loading. The crack length during the tests was monitored by direct current potential drop, which was helpful for defining the applied K corresponding to the onset of subcritical crack growth (KIEAC). Additionally, fractographic analysis of KEAC specimens and results from the fracture toughness evaluation of this material in air at room temperature have also been reported.

Thermally Activated Crack Growth and Fracture Toughness Evaluation of Pipeline Steels Using Acoustic Emission

The article presents an approach to assessing the fracture toughness of structural alloys based on thermally activated crack growth and recording acoustic emission signals. The kinetic and structural features of the stable growth of the initiated crack are estimated using a multilevel acoustic emission model based on the time dependence of the logarithm of the cumulative acoustic emission count. The article provides an evaluation of the stable kinetic constants included in the equation of the thermal fluctuation steps of a crack according to literature sources and using the acoustic emission method. It is shown that parameters such as activation energy, activation area before the crack tip, and the rate of non-activation crack growth are stable and show a satisfactory correspondence between the reference literature and real experiments. The approach does not require a set of laboratory experiments to determine the empirical constants of traditional crack growth rate equations, and it also differs in that it takes into account the unique features of the destruction of a particular specimen or technological equipment and allows for a non-destructive assessment of fracture toughness. The values obtained are conservative. The concentration criterion of destruction requires further investigation.